In addition to magnetic resonance tomography (MR), increasingly widespread use in medical diagnostics has in recent years also been made of positron emission tomography (PET). While MR is an imaging method for showing internal structures of the body and displaying sectional views thereof, PET enables metabolic activities to be visualized and quantified in vivo.
PET exploits the particular characteristics of positron emitters and of positron annihilation to quantitatively determine the functioning of organs or cell regions. Appropriate radiopharmaceuticals that are marked with radionuclides are therein administered to the patient prior to the examination. As they decay the radionuclides emit positrons which after a short distance interact with an electron, as a result of which what is termed an annihilation occurs. This gives rise to two gamma quanta which fly apart in opposite directions (displaced by 180°). The gamma quanta are registered within a specific time window by two PET detector modules located mutually opposite (coincidence measurement), as a result of which the site of the annihilation is determined at a position on the connecting line between said two detector modules.
For detection purposes the detector module must in the case of PET generally cover much of the length of the gantry arc. The module is subdivided into detector elements having a lateral length of a few millimeters. Upon detecting a gamma quantum each detector element generates an event record indicating the time and the detection site, which is to say the relevant detector element. This information is conveyed to a high-speed logic array and compared. A gamma decay process on the connecting line between the two associated detector elements is assumed if two events coincide within a maximum time span. The PET image is reconstructed using a tomography algorithm, what is termed back-projection.
It is known to employ combined PET/CT devices for, for example, compensating the deficient spatial resolution of PET systems. CT simultaneously offers a representation of the patient's anatomy so that when the CT and PET data are mutually superimposed it is possible to establish precisely where in the body the PET activity occurred. In combined PET/CT devices a PET device and CT device are typically arranged one behind the other such that the patient can be transferred seamlessly from one device to the other during an examination. The two measurements can then be performed in direct succession.
It is advantageous to combine a PET device with an MR device because MR offers a higher soft tissue contrast than CT. Combined MR/PET systems are already known in which the PET detectors are arranged within an opening defined by the MR magnet together with the gradient system and exciting coil. They are therein positioned next to the exciting coil so that the examination volumes of the MR and PET system do not coincide but are offset in the z direction. Analogously to the PET/CT system it is consequently not possible here to measure PET and MR data simultaneously.
In this case it is particularly preferred for the PET device to be arranged inside the MR device and for the two examination volumes to be mutually superimposed. It will then be possible to ascertain both morphological MR data and PET data during a single measuring operation. Apart from the time-saving impact, both image data sets can be presented in a simple manner, mutually superimposed so that a diagnosis will be made easier for the physician.
For integrating the PET device and MR device it is necessary to arrange the PET detectors inside the MR device so that the imaging volumes will be positioned isocentrically. For example, the PET detectors can be arranged on a support structure (support tube, gantry) located inside the MR device. They can include, for example, 60 detectors arranged annularly on the support tube. Each of the detectors, which can also be combined into detector blocks, requires a connected cooling means and electric supply lines. These must likewise be arranged inside the MR device. A number of signal processing units are additionally required that are likewise arranged inside the MR device. These are connected to the detectors via the electric supply lines and serve for signal processing.
When, though, MR and PET are used jointly in a combined system, the gamma quanta will be attenuated by anything situated between the site of origin of the respective gamma quanta and the PET detector. The attenuation must be taken into account in the reconstruction of PET images so that image artifacts will be avoided. Situated between the site of origin of the gamma quantum in the patient's body and the acting PET detector are tissue within the patient's body as well as air, generally, and a part of the MR/PET system itself, for example a cover of the patient opening or a patient positioning table. The attenuation values of the components or accessory parts requiring to be taken into account are compiled into attenuation maps (μ maps). Thus, for example, an attenuation map can be produced for the patient positioning table. The same applies to, for instance, local coils attached to the patient for MR examinations. In order to produce the attenuation map it is necessary to ascertain and combine the attenuation values. They can be ascertained by means of, for example, a CT recording or PET transmission measurement of the respective component. Attenuation maps of said kind can be measured on a once-only basis because the attenuation values do not change over the life of the respective component.
It is known in the case of PET/CT systems to calculate an attenuation map from CT recordings using the x-ray absorption coefficients and use it to correct the attenuation of PET data. This can also be employed in measuring attenuation values of the components. It is not possible in the case of PET systems to directly ascertain the attenuation map from the actual measurement data. Homogeneous PET phantoms have to be used for measuring so that the intensity of the gamma quanta arising will be known.